In order to convert, for example, an 8-bit digital signal to a corresponding analog signal one may employ a ladder or string of 256 resistors and control switches, which may be arranged in a two dimensional switching matrix, to couple any selected node of the string to an output. The string may be energized by a reference signal, for example a signal defined by two reference voltage sources. A converter or divider of this kind has considerable utility and has the advantage that monotonicity of the output is guaranteed. However, the usefulness of a resistive string converter is limited by the settling time required by the converter and also the variation of settling time with input code, that is to say the value of the bits controlling the switches coupled to the nodes of the string. The problem is particularly acute for converters realised using CMOS technology. The slow settling time is due mainly to the high output impedance of the converter. In a typical arrangement wherein an array of switches is employed to select a node for coupling to an output, the output impedance comprises two conductive switch resistances and the parallel impedance of the two sections of the string defined by that node. This parallel impedance is at its lowest when the selected node is at one end of the string and may be substantially zero for one end of the string and substantially the resistance of one resistive segment at the other end of the string. The corresponding nodes are associated with the highest and lowest possible codes (for example all zeros and all ones). For input codes which are intermediate the extreme ranges, the parallel impedance is very much higher. In particular, for a node at the middle of the resistor string, the parallel resistance is (for an eight bit converter) sixty-four times the resistance of one segment, because it is the effective impedance of two 128-resistor strings in parallel.
To improve the settling time of the converter, the value of the resistive segments ought to be decreased but it is not practical to obtain very low values of resistors generally in integrated circuit technology and particularly in CMOS technology.
Moreover a decrease in the value of the resistors makes matching of resistors more difficult and therefore adversely affects linearity and increases power dissipation.
It is accordingly the object of the invention to provide an improved resistive string converter.
A more particular object of the invention is to provide a resistive string converter which is associated with at least one and preferably two coarse ladders which are connected in parallel with the main resistive string and which accurately define voltage segments across the resistive string and shunt the high values of resistance for mid range selections.
Other objects and advantages of the invention will become apparent during the following description of a preferred embodiment with reference to the drawings.